The present invention relates to a mould driving device.
The following description makes specific reference, although without limiting the scope of the invention, to moulds for the production of containers such as bottles or the like made of polyethylene, polypropylene or other plastic materials.
In general, production of the above-mentioned type of containers involves the extrusion of a continuous cylindrical tube of plastic material towards a forming station housing a mould designed to give the containers the required shape. Moulding is performed by closing a piece of tube between two half-shells of the mould and, on one hand, seals the tube along the profiles of the half-shells and, on the other, cuts off the excess material, again along the profiles of the half-shells, which, in this way, act as compression cutting edges. Depending on the material used and the length of the cutting line, the forces to be applied in order to close the half-shells vary from several tons to several tens of tons.
Traditionally, amongst mould driving devices, such forces are guaranteed directly, or by means of linkages, by double-acting hydraulic cylinders with maximum operating pressures normally of between 100 and 200 atmospheres.
As well as guaranteeing the high closing forces required, mould driving devices must guarantee mould closing in two separate steps, specifically in a step in which the half-shells rapidly approach one another, braking when they reach the final stretch of the stroke, with subsequent slow contact between the two half-shells for the sealing, and a rapid closing step for the half-shells, commonly known as bumping, to cut off waste with impulsive compression.
For this reason, the mould driving device hydraulic cylinders are supplied by relatively complex hydraulic circuits.
Driving devices with hydraulic cylinders are very inefficient, and therefore require quite a lot of energy in order to operate. Moreover, they are noisy, they need extra energy to cool the oil which, during the compression steps, tends to overheat, and they are dangerously polluting due to the large quantity of oil they require in their supply circuits.